Fine tuning tankless water heater on well system

I recently undertook installing a Navien tankless in my home. Its a cr240a. The box itself seems to work. I have checked the gas pressure (11-13 column inches) and the water pressure, on a well, (40-60psi). It works well enough with one shower on although its warmer and cooler with the cycling of the well pump. I also notice that when more than one person takes a shower at a time, the hot water pressure drops significantly, especially in the upstairs bathroom. I think there may be 2 different problems and wondering the best solution. One is the variable water pressure. I wonder if a stop cycle valve is the the way to go. The other problem is the cold water pressure overcoming the hot water when more than one appliance is on. Is a tempering shower valve the best solution for this or is there someother fix? I bought the larger unit believing it would power 2 showers at one time, but apparently there is more complexity to this concept. Thanks, Jim

I aproached it from several angles. It has a 2 stage regulator. 3/4 pipe from the tank to the house at high pressure. Then 3/4 pipe about 8 feet from the 2nd stage regulator to the tankless heater. I maintain and install the gas kilns for our tile business, and according to my piping book, this should supply 329000 btu. I also installed a pressure gauge on the gas line before the heater and it shows the pressure to be 11.5 column inches while the heater is on full bore. This should be more than enough supply, unless I am missing something. The line used to supply a hot water heater tank and a 125000 btu furnace, so the regulator should be able to handle the supply. That is my only uncertainty is the regulator. It is older, but the gauge says the pressure is there! Don't know the inlet temp. I'll check it out. Output is set at 120f. Jim

cold water temp is 55 f
I changed the output to 125f. I can now can get 120f at the bath faucet.
I tested the gas pressure again. With 2 bathtub faucets on, and 3 sinks on the gas pressure is 10.75 column inches. With 5 hot water facets on the water flow is very low. I am going to check how many gpm the cold water is bringing to the house.

I'm no expert but the guy who installed mine put it right by the gas regulator and said the length of the gas line makes all the performance difference. For all I know he could have been FOS.

I do know from testing mine increasing my water pipes to 3/4 made a big difference. I also noticed you can use more showers if you lower the temp to say 105'.

I do agree with you that yours appears to be limiting somewhere. I would say lower the temp to 105 and if you see more pressure then you know the problem is in the tankless. I'd call the manufacturer to see what they say.

The guy is right you do want it as close as possible. I think I am starting to understand. You set the hot water at the temp you want to shower then just use all hot water. I think I'll put the remote in the bathroom so I can set it when we shower and fill the tub. I also might try the pressure valve thingy for the well pump pressure. Thanks for helping me think about this. Jim

Fluctuating pressures have always been a problem for tankless causing temp fluctuations and the pressure drop across the heat exchangers causes the cross connection migration of cold to hot. With the Navien and because of it's design in order to obtain the highest efficiency out there the problems you have witnessed is compounded.

You don't mention what type of well system you have and if your capable of increasing operating pressure. If you can, and the well will produce, that is the first step. Then depending on a bit of your tank size, closing down on the on/off differential will help also

The best think and if the well can produce is raise it as high as you can, 50-60, as long as it will shut off and install a pressure reducing valve set at the low end pressure or no more than 5 PSI above. Now you have consistent and constant pressure to the home and heater. The cross connection/migration start to eliminate or lessen as pressure rises.

Sounds like tankless is just a poor fit to a typical well installation. It's been 20 years since I was on a well, but I do recall troubleshooting a failed pressure tank and failed pressure switches.

Apparently one needs much tighter control of supply pressure for tankless than is typical on a well. Varying temps and such are a real deal killer for tankless.

If one has to add a PRV to the system another $65 for the part and X for the installation must be added to the overall install cost. Plus the PRV has a limited life, so plan on replacing it at about the same frequency as the heater or the pressure tank.

here is an interesting note:
The flow rate of a faucet or shower head is measured in GPM and is a critical factor in sizing a tankless water heater.

The tub fill flow is about 5 gpm. The shower head is 3 gpm

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You say one shower is 3 gpm so two would be 6 gpm. At 77 degree rise you output 5 gpm. That's proving your input water temp is exactly 55 in the morning. From a well???

Also, the chart I just looked at here says you will get 3.2 gpm with 55 degree ground water and a 2.5 gpm shower head.

I think you may be operating just about right. I know it sucks but I was noticing that I can't run two showers effectively with the temp at 125 degrees (my normal setting). It was about 40 degrees outside this morning but don't know about the input water temp. Luckily we shower seperately but the good news is I get plenty of hot water now (I'm last after two women) which I didn't always get with the tank.

cold water temp is 55 f
I changed the output to 125f. I can now can get 120f at the bath faucet.
I tested the gas pressure again. With 2 bathtub faucets on, and 3 sinks on the gas pressure is 10.75 column inches. With 5 hot water facets on the water flow is very low. I am going to check how many gpm the cold water is bringing to the house.

Click to expand...

How often do you run all your hot water facets at the same time? That test is fine for the gas pressure test.

Maybe you should try new aereators and new shower heads before you go too far.

Also, the chart I just looked at here says you will get 3.2 gpm with 55 degree ground water and a 2.5 gpm shower head.

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I believe that you are misreading (or maybe just misstating) that chart. It does not give the gpm. It gives the # of 2.5 gpm showerheads that the unit can support heat input wise. However, that's not really possible in the real world either because the chart at the end shows the unit would take over 40 psi of pressure drop at that flow rate.

On the other hand, you appear to be correct about the real world capability of the unit. Realistically looking at the two charts and recognizing that the well pressure is probably varying between 40-60 psi, the unit can probably only support about 3.5 gpm total throughput for showering (10 psi pressure drop) without becoming too variable.

I wonder if the 3 gpm the OP reported is measured? The 2.5 gpm's I've tested came in at about 2.3 gpm across a wide pressure range.

Luckily we shower seperately but the good news is I get plenty of hot water now (I'm last after two women) which I didn't always get with the tank.

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With a 50 gallon tank @ 120-125 F and 1.5/1.6 gpm showerheads we never run out of hot water showering. I've not tried three simultaneous showers, but two simultaneous is a common mode. Back when we still had 2.5 gpm nominal showerheads two simultaneous was fine, but if we ran the dishwasher at the same time (~5 gallon all hot draw) the showers would begin to cool off near the end...I did that intentionally to keep the kids from taking very long showers.

I agree with those suggesting the OP try low flow showerheads for this application. I'm using HighSierra's and Roadrunners.

With a 50 gallon tank @ 120-125 F and 1.5/1.6 gpm showerheads we never run out of hot water showering. I've not tried three simultaneous showers, but two simultaneous is a common mode. Back when we still had 2.5 gpm nominal showerheads two simultaneous was fine, but if we ran the dishwasher at the same time (~5 gallon all hot draw) the showers would begin to cool off near the end...I did that intentionally to keep the kids from taking very long showers.

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My tank was in a crawl space and was a gas low boy roughly 3 ft tall but kind of fat. I am sure it wasn't 50 gal's.

You all have given me some info to chew on. I have found if I turn the temp down to 105 and have hot water only it gives good pressure in the shower with only slight temp variation. We will be remodeling the bathrooms and I'll get low flow fixtures. I didn't realize that tankless is such a different animal but I think we can live with it.

A tankless heater should have a spec on GPM vs. temp rise since it heats the water on the fly. A tank heater doesn't need it.
1 GPM = 8.3 # of water per minute and requires 8.3 BTU for each 1F rise above incoming water. A 70F rise would need 580 BTU/min = 35,000 BTU/hr.

We added onto our house and didn't have the money to do anything with our how water system yet. I've been very surprised to see our TK-2 keep up. I'm still planning on some changes soon but it seems to be working for the moment.

I'm no expert but the guy who installed mine put it right by the gas regulator and said the length of the gas line makes all the performance difference. For all I know he could have been FOS.

I do know from testing mine increasing my water pipes to 3/4 made a big difference. I also noticed you can use more showers if you lower the temp to say 105'.

I do agree with you that yours appears to be limiting somewhere. I would say lower the temp to 105 and if you see more pressure then you know the problem is in the tankless. I'd call the manufacturer to see what they say.

Click to expand...

I agree here as well. From my (not) vast experience with Plumbing and repairs, I had my main run pipes all sized up to 3/4" and 1/2" runs off those to each device. It seems to have made the 2 showers and the LG front loader to get all they need and a minimal pressure drop (well supplied, large bladder tank).

The LG front loader is a different story though. Its filling procedure is driving the tankless crazy..... on/off, on/off, many times in the short fill period of the washer. But, thats for another post